Primary cells are designed to be used once and discarded. Generally speaking, the electrochemical reactions occurring in the cells are not reversible: As a primary cell is used, the reactions therein use up the chemicals that generate power and irreversible reaction products.
Secondary cells facilitate reversible cell reactions that allow them to recharge, or regain their cell potential, through the work done by passing currents and converting the products back to reactant status. As opposed to primary cells that experience irreversible electrochemical reactions such as gassing, secondary cell reactions can be reversed allowing for numerous charges and discharges.